Three-time DTM champion Rast has made a long-awaited comeback to the series this year, joining the factory-supported Abt Sportsline team alongside GT3 specialists Kelvin van der Linde and Ricardo Feller.
But the German driver returned empty handed from his first weekend back in the series in Portugal, suffering a DNF in the opening race and finishing an underwhelming 12th in the second 33-lap contest on Sunday.
Qualifying 13th for the second race with a time that was only three tenths shy of the front row, Rast made some progress in the first part and climbed to 11th behind his teammate Feller in the sister Audi R8 LMS GT3.
But after completing his mandatory pitstop on lap 8, Rast dropped behind the GruppeM Mercedes of Maro Engel and was unable to pass a car that was much quicker on the straights, and eventually dropped behind the SSR Porsche of Dennis Olsen.
This was the first time Rast failed to score a point in a DTM round since Norisring in 2018, when all Audis struggled for pace and Rast lost more than 15 seconds in the pits with a botched tyre change.
“It was a difficult weekend for me and unfortunately, I went home without points," he said. "On Saturday, I had a pretty good qualifying session by my standards: I was on the same level as the other Audi drivers.
"Unfortunately, I didn’t score any points in the race due to the unfortunate accident at the restart."
In an interview with Motorsport.com's sister title Motorsport-Total.com, he added: “Difficult day [on Sunday], especially the race.
"Once you're in the train, there's nothing you can do. You can't overtake, it's very difficult to get close to the competition because on the straights they drive away from you and in the corners you're in line. But we're always much too far away to make a manoeuvre.
“That's why you can only pull off a manoeuvre with all your might. That doesn't make much sense at the back in P10 or P11.
“Maro Engel and Dennis Olsen were the two guys I was fighting with the whole race. But coming out of the last corner, the Mercedes makes up 50 metres on you. Then you start again, and on the straight he pulls away again, and so it goes back and forth. A bit frustrating.
“Because of the BoP, each car has different strengths in different areas. Not every car can be equally good everywhere. We're good in the corners, but on the straights we're lacking a bit here and there. That makes it difficult for us, of course.”
Rast was the top qualifier within the Abt squad on Saturday in 11th, well clear of Feller and van der Linde, and trailed the leading Audi of Team Rosberg driver Nico Muller by just 0.074s.
However, the tables turned on Sunday when he qualified nearly half a second off polesitter Muller, and was even outpaced by the second Team Rosberg car of Dev Gore.
Muller later went on to dominate the race from pole position, securing his first DTM victory since the 2020 title decider at Hockenheim.
The results on Sunday gave the impression that Team Rosberg has made a major breakthrough after enduring a tough 2021 season, while Abt has dropped down the pecking order after challenging for the title last year.
Rast said he was happy to see his former title rival Muller return to winning ways in the DTM, but had no explanation for the contrasting fortunes of the two Audi squads at Portimao.
“You can only say congratulations to Nico,” said Rast. “He had a sensational day. The team has also improved a lot from last year to this year.
“Last year it was the other way around, and now Rosberg has really worked a lot over the winter. The car and Nico really did a great job this weekend and also deserved to win the race.”
He added: “Difficult. You can't say what the reason is. You'd have to have the data for that, which we don't have, of course.
“You'd have to look into it, but now it's up to us to catch up at the next test and try to maybe go in a different direction.
“But last year in Monza it was also the case that Nico wasn't that bad. Maybe it's just our car not matching the track. We'll see at the next race.”